The present invention relates to an acceleration sensor and a manufacturing method for the same, and more specifically, a mass production type acceleration sensor to be used for small-sized household electronic devices and a manufacturing method for the same.
Household electronic devices with a microprocessor installed inside, such as cell phones, digital cameras, electronic games, and PDAs have strongly spread, and recently, the demand for acceleration sensors to be installed in these electronic devices or input devices of electronic devices has also increased. In an electronic device having an acceleration sensor installed inside, acceleration components with an impact or a vibration applied to the device main body can be taken in the microprocessor as digital data, so that proper processing while grasping the physical environments around the electronic device becomes possible. For example, in the case of a digital camera, correction for camera shake can be carried out by detecting the acceleration that has acted at the moment when the shutter button is pressed. Furthermore, in an input device of an electronic game, it also becomes possible to input a user""s operation instruction in the form of an acceleration. For example, when an acceleration sensor is used for detection of an gravity acceleration, the acceleration sensor can be used as a clinometer, and it becomes possible to recognize the azimuth of the entire electronic device (degree of inclination from the horizontal status), so that predetermined operation and input become possible based on a user""s operation for inclining the entire electronic device.
It is desirable that an acceleration sensor to be installed inside such a small-sized household electronic device is small-sized and suitable for mass production. For example, Japanese Patent Publications No. 01-263576 and No. 03-202778 disclose structures of small-sized acceleration sensors suitable for mass production, and Japanese Patent Publication No. 04-249726 discloses a manufacturing method for mass production of such an acceleration sensor by using a silicon substrate. As such a general acceleration sensor, a type has been generally used in which a weight is joined to a flexible substrate, and the substrate is bent in response to an acceleration applied to the weight, and the bend of this substrate is electrically detected. For detecting the bend of the substrate, various detectors such as a piezo resistor element, a capacitor element, or a piezoelectric element have been used.
As mentioned above, as a small sized acceleration sensor suitable for mass production, a sensor using a semiconductor substrate such as a silicon substrate has been proposed. In order to increase the detection sensitivity in such an acceleration sensor, an increase in weight mass or an increase in flexibility of the weight supporting portion is required. However, in all cases where an excessive acceleration is applied, there is a possibility that the weight supporting portion is broken. Particularly, if the sensor casing drops by mistake, a great impact is applied to the weight, and there is a possibility that the weight supporting portion formed of a semiconductor is damaged. Therefore, it is required to provide a physical control structure for controlling displacements of the weight within a predetermined range under normal conditions. Also in the case of the acceleration sensors disclosed in the abovementioned publications, a physical control structure such as a control substrate or a pedestal is provided for controlling vertical and horizontal displacements of the weight. When an excessive acceleration acts, a part of the weight comes into contact with the control substrate or the pedestal, and this suppresses displacements of the weight within a predetermined range of freedom. Therefore, the weight supporting portion can be prevented from being applied with an excessive stress and being broken.
However, it is required that such a control structure has a predetermined shape and is disposed at a predetermined position in accordance with the shape and disposition of the weight. Therefore, in order to manufacture an acceleration sensor with a control structure, an etching step or machining step is additionally required, and this makes the manufacturing process complicated. Particularly, in order to secure uniform performance among lots to be manufactured as mass-produced items, it is required that the distance between the weight and the control structure is accurately set. Therefore, in the conventional acceleration sensors disclosed in the abovementioned publications, a great technical burden is imposed on the step for forming the control structure, and this also poses a problem in terms of cost reduction.
An object of the invention is, therefore, to provide an acceleration sensor in which an accurate control structure for controlling displacements of the weight can be easily constructed.
(1) The first feature of the present invention resides in an acceleration sensor comprising:
a substrate layer having a displacing section provided at a central portion, a fixed section provided around the displacing section, and a connecting portion for connecting the displacing section to the fixed section so that the displacing section can displace;
a weight disposed under the substrate layer, the weight including an upper surface peripheral portion that opposes a control surface formed on a lower surface of an inner part of the fixed section;
a pedestal which is disposed so as to surround the weight and supports and fixes the fixed section from below;
a weight joint layer which is interposed in a region other than the upper surface peripheral portion in a space between the weight and the substrate layer and is made of a material different from that of the weight and the substrate layer, and connects the weight to a lower surface of the displacing section; and
a displacement detector for detecting a displacement of the displacing section; wherein
when an acceleration is applied to the weight, the displacing section displaces with respect to the fixed section due to bending of the connecting portion, and a thickness of the weight joint layer is set so that the upper surface peripheral portion comes into contact with the control surface to control displacement when a degree of a predetermined directional component of an applied acceleration exceeds a predetermined tolerance.
(2) The second feature of the present invention resides in an acceleration sensor according to the first feature, wherein:
a pedestal joint layer made of the same material as that of the weight joint layer is provided between the pedestal and the fixed section, and the pedestal and the fixed section are joined together by the pedestal joint layer.
(3) The third feature of the present invention resides in an acceleration sensor according to the second feature, wherein:
a material for forming the substrate layer, the pedestal, and the weight and a material for forming the weight joint layer and the pedestal joint layer are different in etching property from each other.
(4) The fourth feature of the present invention resides in an acceleration sensor according to the first to the third features, wherein a slit is formed in the substrate layer so that portions of the substrate layer serve as a displacing section, a fixed section, and a connecting portion.
(5) The fifth feature of the present invention resides in an acceleration sensor according to the fourth feature, wherein:
a plurality of slits that have an annular shape partially including an aperture are formed so that apertures face a center of the substrate layer; and
a structure with a fan shape including blade portions surrounded by the respective annular slits is regarded as a displacing section, a portion formed between a pair of slits adjacent to each other is regarded as a connecting portion, and an outer peripheral portion of the substrate layer positioned more outwardly than an outer envelope of the plurality of slits is regarded as a fixed section.
(6) The sixth feature of the present invention resides in an acceleration sensor according to the fifth feature, wherein:
four slits which are formed along contours of approximate squares and have the same annular shape including an aperture at a portion corresponding to one apex of a square are symmetrically formed on the substrate layer that is approximate square.
(7) The seventh feature of the present invention resides in an acceleration sensor according to the fifth or sixth feature, wherein:
a contour of an upper surface of the weight has a shape which is obtained by outwardly expanding a contour of the displacing section so that displacement of the upper surface peripheral portion of the weight is controlled by the control surface in a region outside and close to an outer envelope of the plurality of slits on a lower surface of the substrate layer.
(8) The eighth feature of the present invention resides in an acceleration sensor according to the seventh feature, wherein:
the weight joint layer includes a central portion joint layer for joining the displacing section and the weight at a central portion of the fan shape and blade portion joint layers for joining the displacing section and the weight at respective blade portions of the fan shape.
(9) The ninth feature of the present invention resides in an acceleration sensor according to the first to the eighth features, wherein:
a distance between a side surface of the weight and an inner surface of the pedestal is set so that displacement of the weight is controlled by contact of the side surface of the weight with the inner surface of the pedestal when a degree of a predetermined directional component of an applied acceleration exceeds a predetermined tolerance.
(10) The tenth feature of the present invention resides in an acceleration sensor according to the first to the ninth features, wherein:
a thickness of the weight is set so that a bottom surface of the weight is located above a bottom surface of the pedestal with a predetermined distance, and when the pedestal is fixed on a control substrate, the predetermined distance is secured between the bottom surface of the weight and the upper surface of the control substrate; and
the predetermined distance is set so that displacement of the weight is controlled by contact of the bottom surface of the weight with the upper surface of the control substrate when a degree of a predetermined directional component of an applied acceleration exceeds a predetermined tolerance.
(11) The eleventh feature of the present invention resides in an acceleration sensor according to the first to the tenth features, wherein:
the displacement detector includes piezo resistor elements disposed on the connecting portion and a detecting circuit for detecting electric resistance changes of the piezo resistor elements.
(12) The twelfth feature of the present invention resides in an acceleration sensor according to the eleventh feature, wherein:
a left connecting portion is provided at a left of the displacing section and a right connecting portion is provided at a right of the displacing section, two piezo resistor elements are disposed on the left connecting portion and two piezo resistor elements are disposed on the right connecting portion so that a total of four piezo resistor elements are aligned roughly in line, and the detecting circuit includes bridges using the four piezo resistor elements.
(13) The thirteenth feature of the present invention resides in an acceleration sensor according to the first to the tenth features, wherein:
the displacement detector includes an auxiliary substrate disposed at a predetermined distance above the substrate layer, a displacing electrode formed on an upper surface of the displacing section, a fixed electrode formed on a lower surface of the auxiliary substrate, and a detecting circuit for detecting changes in capacitance of a capacitor element formed by the displacing electrode and the fixed electrode.
(14) The fourteenth feature of the present invention resides in an acceleration sensor according to the thirteenth feature, wherein at least an upper surface region opposing the fixed electrode, of the displacing section has conductivity, and the displacing section itself is used as a displacing electrode.
(15) The fifteenth feature of the present invention resides in a manufacturing method for the acceleration sensor according to the second feature, comprising:
a preparation step for preparing a material substrate including laminated three layers of a first layer, a second layer, and a third layer in order from the upper side;
a substrate layer forming step for forming a substrate layer including a displacing section, a fixed section, and a connecting portion by forming a slit in the first layer by etching predetermined regions of the first layer in a thickness direction until an upper surface of the second layer is exposed according to an etching method that has erodibility for the first layer and does not have erodibility for the second layer;
a weight/pedestal separating step for separating the third layer into a weight and a pedestal by etching predetermined regions of the third layer in a thickness direction until a lower surface of the second layer is exposed according to an etching method that has erodibility for the third layer and does not have erodibility for the second layer; and
a joint layer forming step for forming a weight joint layer and a pedestal joint layer by remaining portions after etching the second layer in a thickness direction and a layer surface direction from exposed portions of the second layer according to an etching method that has erodibility for the second layer and does not have erodibility for the first and third layers.
(16) The sixteenth feature of the present invention resides in a manufacturing method for the acceleration sensor according to the fifteenth feature, wherein:
in the substrate layer forming step, a plurality of slits that have an annular shape partially including an aperture are formed so that the aperture faces a center of the first layer; and
a displacing section is formed by a structure having a fan shape that includes blade portions surrounded by the respective annular slits, a portion between a pair of slits adjacent to each other is formed as a connecting portion, and a fixed section is formed of an outer peripheral portion of the first layer positioned more outwardly than an outer envelope of the plurality of slits.
(17) The seventeenth feature of the present invention resides in a manufacturing method for the acceleration sensor according to the sixteenth feature, wherein:
in the weight/pedestal separating step, etching for separating a weight and a pedestal is carried out at a position more outward than the outer envelope of the plurality of slits, so that a weight having a contour expanding more outwardly than a contour of the displacing section is formed, and displacement of the upper surface peripheral portion of the weight can be controlled by a control surface formed in a region outside and close to the outer envelope on a lower surface of the first layer.
(18) The eighteenth feature of the present invention resides in a manufacturing method for the acceleration sensor according to the fifteenth to the seventeenth features, further comprising:
a thickness adjusting step for making a thickness of the weight smaller than a thickness of the pedestal by removing a lower layer portion of a region where the weight is formed of the third layer by etching; and
a control substrate joining step for joining a control substrate to a bottom surface of the pedestal.
(19) The nineteenth feature of the present invention resides in a manufacturing method for the acceleration sensor according to the fifteenth to the eighteenth features, wherein:
the first layer and the third layer are made of the same material.
(20) The twentieth feature of the present invention resides in a manufacturing method for the acceleration sensor according to the nineteenth feature, wherein:
the first layer and the third layer are made of silicon, and the second layer is made of silicon oxide.
(21) The twenty-first feature of the present invention resides in a manufacturing method for the acceleration sensor according to the fifteenth to the twentieth features, wherein:
an induced coupling plasma etching method is used for etching in the thickness direction in the substrate layer forming step and the weight/pedestal separating step.